Giardia lamblia is a major cause of diarrheal disease worldwide, yet little is known of its mechanisms of pathogenesis. Trophozoites are not known to invade, express any virulence factor, or elicit an inflammatory response. Therefore, it is crucial to understand giardial interactions with the intestinal epithelium from both the parasite and the host perspective. The overall goal of this Unit is to elucidate the molecular, cellular, and biochemical cross talk between giardial trophozoites and human intestinal epithelial cells in vitro and in a human intestinal xenograft model. The underlying hypothesis is that physiologic stimuli from host epithelial cells may modulate giardial behavior and enable trophozoites to better colonize and evade natural host defenses. Conversely, attachment of trophozoites may elicit physiologically relevant responses to the host epithelial barrier. We will determine whether the cross talk is due to giardial attachment to live cells or caused by factors released into the environment. Aim 1 is to investigate trophozoite survival., growth, and attachment responses to cultured human intestinal epithelial cells. Aim 2 is to test the hypothesis that exposure to epithelial cells or signals from them may: A. cause encysting trophozoites to revert from differentiation to growth; B. change overall trophozoite development programs as manifested by alterations in protein, protein phosphorylation, and mRNA fingerprints; C. induce expression of giardial virulence factors. Aim 3 is to test the hypothesis that attachment will induce changes in the cytoskeleton and ultrastructure of both trophozoites and intestinal epithelial cells that may give key insights into their interactions. Aim 4 is to evaluate the human intestinal xenograft model as a system to study the response of normal intestinal epithelial cells to giardial colonization. This system will allow us to define both host and parasite factors required for infection. These studies will greatly extend our understanding of an important intestinal parasite and have the potential to yield new insights into intestinal barrier and defense functions.